Stamp dealers used to exchange stamps by the hundreds and thousands. Here's a letter written in English by J. G. Paquelet at the turn of the century to a dealer in Canada:
Luxemburg June 6th '98
Your price list July in hand. Please send me 1000 or 2000 Canadian stamps, 10 to 20 varieties equally mixed and you will receive 1000 or 2000 Luxemburg stamps in same mixture. All stamps in good condition.
I should be glad to have 500 to 1000 South and Central America and Newfoundland in exchange for better values of Luxemburg.
Yours very truly
J. G. Paquelet
Comptoir de Timbres to Luxembourg
Luxembourg-Ville, June 6, 1898 to London, Ontario, Canada,
received June 18, 1898.
And here's his 1898 buying list, which he sent worldwide (and in this instance, 10 days after sending a card to A. F. Wicks, Esq. in Canada).
You'll notice he was offering 1000 "Cartes postales usées, 5 valeurs, le 100, 1 fr. 75; le 1000, 12 fr." I'll take 10,000!
Luxembourg-Ville, June 16, 1898 to Colchester, England,
sent at the five-centime UPU printed matter rate